1. Field of the Invention
The present invention is related to the development of Nitrogen face (N-face) nitride based millimeter (mm) wave transistors with high power and high efficiency, through achieving low parasitic resistances and orders of magnitude reduction in leakage.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [Ref. x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Gallium nitride (GaN) devices have been shown to be promising for high voltage high frequency applications, due to the high breakdown field and high electron velocity in GaN, as well as high charge density in the channel. Growth of a high quality and reliable semi-insulating buffer is essential for low DC dissipation and sharp pinch-off of high electron mobility transistors (HEMTs).
Unintentionally-doped (UID) gallium face (Ga-face) GaN, grown directly on silicon face (Si-face) silicon carbide (SiC), exhibits varied degrees of n-type conductivity. Silicon (Si) incorporated into the UID GaN from the SiC substrate, which becomes a shallow donor, is a major source of free carriers based on secondary ion mass spectroscopy (SIMS) measurements. A successful approach to suppress buffer leakage was the growth of a large bandgap aluminum nitride (AlN) nucleation layer prior to growing the buffer [Ref. 1]. The AlN isolates the GaN from the SiC substrate and prevents Si incorporation into the buffer. Acceptor doping by iron [Ref 2] or carbon [Ref 3] have also been employed to compensate unintentional shallow donors (such as oxygen) in GaN.
The performance of highly-scaled nitride-based HEMTs is limited by parasitic resistances at the ohmic contacts. In devices grown on the Ga-polar or (0001) orientation, multiple technologies such as n+ cap layers, ion implantation, multi-channels and regrown ohmic regions have been used to obtain lower ohmic contact resistance. In Ga-face HEMTs, however, the presence of a large bandgap barrier cap, as well as a conduction band discontinuity between the two-dimensional electron gas (2DEG) and the ohmic metal, introduce inevitable challenges for further reduction of the contact resistance.